Method for calibrating optical detector operation with marks formed on a moving image receiving surface in a printer

ABSTRACT

A method for calibration of an optical sensor to scan printed marks in a printer includes operating inkjets to form printed marks on an image receiving surface and activating the optical sensor after the image receiving surface moves a predetermined distance to generate scanned image data of a region of the image receiving surface that is longer than the region containing the printed marks. The method includes identifying an error between the location of the printed marks in the scanned image data and a predetermined expected location for the marks, and adjustment of the distance that the image receiving surface moves prior to activation of the optical sensor to correct the error.

TECHNICAL FIELD

This disclosure relates generally to printers and, more specifically, toinkjet printers that use scanned image data for printhead registrationand detection of missing inkjets.

BACKGROUND

Inkjet printers operate a plurality of inkjets in each printhead toeject liquid ink onto an image receiving surface. The ink can be storedin reservoirs that are located within cartridges installed in theprinter. Such ink can be aqueous ink or an ink emulsion. Other inkjetprinters receive ink in a solid form and then melt the solid ink togenerate liquid ink for ejection onto the imaging member. In these solidink printers, the solid ink can be in the form of pellets, ink sticks,granules, pastilles, or other shapes. The solid ink pellets or inksticks are typically placed in an ink loader and delivered through afeed chute or channel to a melting device, which melts the solid ink.The melted ink is then collected in a reservoir and supplied to one ormore printheads through a conduit or the like. Other inkjet printers usegel ink. Gel ink is provided in gelatinous form, which is heated to apredetermined temperature to alter the viscosity of the ink so the inkis suitable for ejection by a printhead. Once the melted solid ink orthe gel ink is ejected onto the image receiving surface, the ink returnsto a solid, but malleable form, in the case of melted solid ink, and togelatinous state, in the case of gel ink.

A typical inkjet printer uses one or more printheads with each printheadcontaining an array of individual nozzles through which drops of ink areejected by inkjets across an open gap to an image receiving surface toform an ink image. The image receiving surface can be a continuous webof recording media, a series of media sheets, or the image receivingsurface can be a rotating surface, such as a print drum or endless belt.Images printed on a rotating surface are later transferred to recordingmedia by mechanical force in a transfix nip formed by the rotatingsurface and a transfix roller. In an inkjet printhead, individualpiezoelectric, thermal, or acoustic actuators generate mechanical forcesthat expel ink through an aperture, usually called a nozzle, in afaceplate of the printhead. The actuators expel an ink drop in responseto an electrical signal, sometimes called a firing signal. Themagnitude, or voltage level, of the firing signals affects the amount ofink ejected in an ink drop. The firing signal is generated by aprinthead controller with reference to image data. A print engine in aninkjet printer processes the image data to identify which inkjets in theprintheads of the printer are operated to eject a pattern of ink dropsat particular locations on the image receiving surface to form an inkimage corresponding to the image data. The locations where the ink dropslanded are sometimes called “ink drop locations,” “ink drop positions,”or “pixels.” Thus, a printing operation can be viewed as the placementof ink drops on an image receiving surface with reference to electronicimage data.

In order for the printed images to correspond closely to the image data,both in terms of fidelity to the image objects and the colorsrepresented by the image data, the printheads are registered withreference to the imaging surface and with the other printheads in theprinter. Registration of printheads refers to a process in which theprintheads are operated to eject ink in a known pattern and then theprinted image of the ejected ink is analyzed to determine the relativepositions of the printheads with reference to the imaging surface andwith reference to the other printheads in the printer. Operating theprintheads in a printer to eject ink in correspondence with image datapresumes that the printheads are level with one another across a widthof the image receiving surface and that all of the inkjets in theprinthead are operational. The presumptions regarding the positions ofthe printheads, however, cannot be assumed, but must be verified.Additionally, if the conditions for proper operation of the printheadscannot be verified, the analysis of the printed image should generatedata that can be used either to adjust the printheads so they betterconform to the presumed conditions for printing or to compensate for thedeviations of the printheads from the presumed conditions.

During operation, one or more inkjets in the printheads may becomeinoperable. An inoperable inkjet includes any inkjet that fails to ejectink drops on demand, ejects ink drops only intermittently, or ejects inkdrops onto an incorrect location on the image receiving surface.Inoperable inkjets in a print zone can produce defects and artifacts inprinted images. Some printers detect inoperable inkjets during a printjob and compensate for the inoperable inkjets until the printheadscontaining the inoperable inkjets are cleaned or serviced. Scanned imagedata from printed patterns that are formed on the image receivingsurface are used for both registration of the printheads and foridentification of inoperable inkjets.

Analysis of printed images is performed with reference to twodirections. “Process direction” refers to the direction in which theimage receiving surface is moving as the imaging surface passes theprinthead to receive the ejected ink and “cross-process direction”refers to an axis that extends across the width of the image receivingsurface, which is perpendicular to the process direction. In order toanalyze a printed image, a test pattern needs to be generated in amanner that enables determinations to be made as to whether the inkjetsoperated to eject ink did, in fact, eject ink and whether the ejectedink landed where the ink would have landed if the printhead waspositioned correctly with reference to the image receiving surface andthe other printheads in the printer. In some printers, an opticalscanner is integrated into the printer and positioned at a location inthe printer that enables the scanner to generate image datacorresponding to the ink image while the image is on media within theprinter or while the ink image is on the rotating image receivingsurface in the printer.

These integrated scanners typically include one or more illuminationsources and a plurality of optical detectors that receive radiation fromthe illumination source that has been reflected from the image receivingsurface. The radiation from the illumination source is usually visiblelight, but the radiation can be at or beyond either end of the visiblelight spectrum. If light is reflected by a white surface, the reflectedlight has the same spectrum as the illuminating light. In some systems,ink on the imaging surface can absorb a portion of the incident light,which causes the reflected light to have a different spectrum. Inaddition, some inks may emit radiation in a different wavelength thanthe illuminating radiation, such as when an ink fluoresces in responseto a stimulating radiation. Each optical sensor generates an electricalsignal that corresponds to the reflected light received by the detector.The electrical signals from the optical detectors are converted todigital signals by analog to digital converters and provided as digitalimage data to an image processor.

In many high-volume printers, the image receiving surface moves past theprintheads and the optical scanner at high speed in the processdirection. For example, some continuous media printers include a mediaweb that moves past the printheads and the optical scanner at a rate ofseveral hundred feet per minute. The optical scanner is only activatedfor brief periods to capture scanned images of the printed test patternson the media web while being deactivated when printed images on themedia web pass the optical scanner. If the scanned image data includeportions of printed images, the registration and inoperable inkjetdetection processes may become less effective since the scanned imagescan be confused with the printed test patterns. Additionally, theprinthead registration and inoperable inkjet detection processes areless effective if the optical scanner only captures a portion of theprinted test pattern. During operation, small changes in the media webincluding slip and web shrinkage introduce small errors insynchronization between the locations of the test patterns on the mediaweb and the optical sensor. As the errors accumulate, the opticalscanner may capture portions of the media web that include the printedimage or may fail to capture the entire printed test pattern.Consequently, improvements to the synchronization of operation for theoptical scanner to enable accurate generation of scanned image data forprinted test patterns would be beneficial.

SUMMARY

In one embodiment, a method calibrates an optical sensor with referenceto image data of scanned marks printed on an image receiving surface ina printer. The method includes operating inkjets in a plurality ofprintheads to form a plurality of marks on a first region of the imagereceiving surface as the first region passes the plurality ofprintheads, the first region having a first length in a processdirection, activating an optical sensor to generate first scanned imagedata corresponding to a second region of the image receiving surface inresponse to the first region of the image receiving surface moving by apredetermined distance in the process direction, the second regionhaving a second length in the process direction that is longer than thefirst length of the first region and at least a portion of the firstregion being contained within the second region, identifying a relativeprocess direction location of the plurality of marks in the firstscanned image data with reference to a first end and a second end of thesecond region in the first scanned image data, identifying an errorbetween the relative process direction location of the plurality ofmarks in the second region and a predetermined relative processdirection location within the second region, adjusting the predetermineddistance in the process direction by an amount corresponding to theidentified error, and storing a value of the adjusted predetermineddistance in a memory to adjust a time of operation for the opticalsensor during generation of additional scanned image data of the imagereceiving surface.

In another embodiment, a different method calibrates an optical sensorwith reference to scanned marks printed on an image receiving surface ina printer. The method includes operating inkjets in a referenceprinthead to form a plurality of marks on a first region of the imagereceiving surface moving past the inkjets, the first region having afirst length in a cross-process axis, activating an optical sensor togenerate first scanned image data of a second region of the imagereceiving surface with a predetermined number of optical detectors thatcorresponds to the second region of the image receiving surface, thesecond region of the surface having a second length in the cross-processaxis that is longer than the first length and the second regionincluding the first region, the first scanned image data furthercomprising a plurality of scanlines with each optical detectorgenerating one pixel in each of the plurality of scanlines, identifyinga relative cross-process axis location of the plurality of marks in thefirst scanned image data with reference to a first end and a second endof the second region in the first scanned image data, identifying anerror between the relative cross-process axis location of the pluralityof marks in the second region and a predetermined relative cross-processaxis location within the second region, identifying a predeterminednumber of pixels in the first scanned image data with a length in thecross-process axis corresponding to the identified error, thepredetermined number of pixels extending in the cross-process axis fromone of the first end and the second end of the first scanned image data,and storing position data corresponding to the predetermined number ofpixels in a memory for use in cropping portions of additional image datagenerated from the optical sensor corresponding to other regions of theimage receiving surface.

In another embodiment, a printer calibrates an optical sensor withreference to scanned marks printed on an image receiving surface in theprinter. The printer includes a media transport configured to move amedia web in a process direction through a print zone and past anoptical sensor, a plurality of printheads in the print zone, eachprinthead including a plurality of inkjets configured to eject ink dropsonto a media web, a sensor operatively connected to a roller in themedia transport that engages the media web, the sensor being configuredto generate a signal corresponding to a length of movement of the mediaweb in the process direction, and a controller operatively connected tothe media transport, the plurality of printheads, the optical sensor,the reflex sensor, and a memory. The controller is configured to operatethe media transport to move a first region of the media web through theprint zone in the process direction, the first region having a firstlength in the process direction, operate the inkjets in the plurality ofprintheads to form a plurality of marks on the first region as the mediaweb moves past the plurality of printheads, activate an optical sensorto generate first scanned image data corresponding to a second region ofthe media web in response to identification that the media web has moveda predetermined distance in the process direction with reference tosignals received from the sensor, the second region having a secondlength in the process direction that is longer than the first length andat least a portion of the first region being contained within the secondregion, identify a relative process direction location of the pluralityof marks in the first scanned image data with reference to a first endand a second end of the second region in the first scanned image data,identify an error between the relative process direction location of theplurality of marks in the second region and a predetermined relativeprocess direction location within the second region, adjust thepredetermined distance of movement for the media web by an amountcorresponding to the identified error, and store a value of the adjustedpredetermined distance in the memory to adjust a time of operation forthe optical sensor during generation of additional scanned image data ofthe media web.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and other features of a printer that is configuredto calibrate the operation of an optical sensor to enable accuratedetection of printed test patterns on a moving image receiving surfaceare described below.

FIG. 1 is a block diagram of a process for adjusting operation of anoptical sensor to generate image data including a test pattern formed onan image receiving surface in a process direction of the image data.

FIG. 2 is a block diagram of a process for adjusting a number of opticaldetectors in an optical sensor that are activated to generate image dataof a printed test pattern in a cross-process direction on the imagereceiving surface.

FIG. 3 is a schematic diagram depicting selected components on a printpath in a printer including a plurality of printheads that form aportion of a printed test pattern on a print medium, and an opticalsensor that generates image data corresponding to the test pattern.

FIG. 4 is a schematic diagram of an inkjet printer that is configured tocalibrate the operation of an optical detector to generate image data ofprinted test patterns that do not include image artifacts from outsidethe printed test patterns.

DETAILED DESCRIPTION

For a general understanding of the present embodiments, reference ismade to the drawings. In the drawings, like reference numerals have beenused throughout to designate like elements. As used herein, the terms“printer” generally refer to an apparatus that applies an ink image toprint media and can encompass any apparatus, such as a digital copier,bookmaking machine, facsimile machine, multi-function machine, etc.,which performs a print outputting function for any purpose. The printerprints ink images on an image receiving surface, and the term “imagereceiving surface” as used herein refers to print media or anintermediate member, such as a drum or belt, which carries an ink imageand transfers the ink image to a print medium. “Print media” can be aphysical sheet of paper, plastic, or other suitable physical substratesuitable for receiving ink images, whether precut or web fed. As used inthis document, “ink” refers to a colorant that is liquid when applied toan image receiving surface. For example, ink can be aqueous ink, inkemulsions, melted phase change ink, or gel ink that has been heated to atemperature that enables the ink to be liquid for application orejection onto an image receiving surface and then return to a gelatinousstate. A printer can include a variety of other components, such asfinishers, paper feeders, and the like, and can be embodied as a copier,printer, or a multifunction machine. An image generally includesinformation in electronic form, which is to be rendered on print mediaby a marking engine and can include text, graphics, pictures, and thelike.

As used herein, the term “process direction” refers to a direction ofmovement of an image receiving surface, such as a print medium orindirect image receiving surface, along a media path through a printer.The image receiving surface moves past one or more printheads in theprint zone to receive ink images and passes other printer components,such as heaters, fusers, pressure rollers, and on-sheet imaging sensors,that are arranged along the media path. As used herein, the term“cross-process” direction refers to an axis that is perpendicular to theprocess direction along the surface of the image receiving surface.

The term “printhead” as used herein refers to a component in the printerthat is configured to eject ink drops onto the image receiving surface.A typical printhead includes a plurality of inkjets that are configuredto eject ink drops of one or more ink colors onto the image receivingsurface. The inkjets are arranged in an array of one or more rows andcolumns. In some embodiments, the inkjets are arranged in staggereddiagonal rows across a face of the printhead. Various printerembodiments include one or more printheads that form ink images on theimage receiving surface. Some printer embodiments include a plurality ofprintheads arranged in a print zone. An image receiving surface, such asa print medium or an intermediate member that holds a latent ink image,moves past the printheads in a process direction through the print zone.The inkjets in the printheads eject ink drops in rows in a cross-processdirection, which is perpendicular to the process direction across theimage receiving surface.

As used herein, the term “test pattern” refers to a predeterminedarrangement of printed marks formed on an image receiving surface by oneor more printheads in the printer. In some embodiments, a test patternincludes a predetermined arrangement of a plurality of marks formed bysome or all of the inkjets in the printheads arranged in the print zoneon a print medium or on an indirect image receiving surface. As usedherein, the term “dash” refers to a printed mark formed on an imagereceiving surface that includes a series of ink drops extending in theprocess direction formed by a single inkjet in a printhead. A dash canbe formed from ink drops located in adjacent pixels in the processdirection on the image receiving surface and can include a pattern ofon/off adjacent pixels in the process direction. As used herein, theterm “pixel” refers to a location on the image receiving surface thatreceives an individual ink drop from an inkjet. Locations on the imagereceiving surface can be identified with a grid-like pattern of pixelsextending in the process direction and cross-process direction axis onthe image receiving surface.

As used herein, the term “reflectance value” refers to a numeric valueassigned to an amount of light that is reflected from a pixel on theimage receiving surface. In some embodiments, the reflectance value isassigned to an integer value between 0 and 255. A reflectance value of 0represents a minimum level of reflected light, such as a pixel that iscovered in black ink, and a reflectance value of 255 represents amaximum level of reflected light, such as light reflected from whitepaper or a bare drum surface used as an image receiving surface. Inother embodiments the reflectance value can be a non-integer value thatcovers a different numeric range. Some embodiments measure reflectancevalues that include multiple numeric values corresponding to differentcolor separations such as red, green, and blue (RGB) values. In a testpattern that includes dashes printed on a highly reflective imagereceiving surface, the image data corresponding to a dash have lowerreflectance values.

As used herein, the term “crop” refers to an image processing operationthat processes pixels of image data in a border region in a manner thateffectively ignores the data content of the pixels in the border regionwhile processing image pixels in a region adjacent to the border regionwith reference only to the data content of the image pixels in theborder adjacent region. For example, a digital controller or other imageprocessing device can ignore selected pixels from the border of an imagewhile processing the pixels in the uncropped central portion of theimage. Crop operations are commonly used in the manipulation of digitalphotographs to remove regions of image data near the edges of the imagedata that include unwanted image artifacts, and to change the relativelocation of features in the uncropped portion of the image compared tothe rest of the image. For example, to adjust the relative location of aprinted mark in a test pattern to the left in the cross-processdirection, the controller crops pixels from the image beginning at theleft border of the image to adjust the relative location of the printedimage in the remaining uncropped image data. Cropping operations includeidentifying a plurality of pixels as being an edge that are not the trueedge of the image, and overwriting the image pixels in a border regionwith a predetermined value that enables the pixel values to be processedwithout affecting the processing of the image pixels in the area ofinterest. For example, the border region can be overwritten with imagedata values that correspond to a bare imaging surface so the imagevalues in the border region cannot be mistakenly identified as marks ina test pattern.

FIG. 4 depicts a continuous web printer 100 that includes four printmodules 102, 104, 106, and 108; a media path configured to transport aprint medium 114 through the printer in a process direction P, acontroller 128, a memory 129, optical sensor 138, and encoders 160 and162. The print modules 102, 104, 106, and 108 are positionedsequentially along the media path and form a print zone for formingimages on a print medium 114 as the print medium 114 travels past theprint modules. The media web travels through the media path in theprocess direction P guided by a pre-heater roller 118, backer rollersexemplified by backer roller 116, and a leveler roller 120. A brushcleaner 124 and a contact roller 126 are located at one end of the mediapath. A heater 130 and a spreader 132 are located at the downstream endof the media path after the media web 114 passes the print modules102-108 in the print zone. After passing through the media path, atakeup-roller (not shown) winds the media web 114 into a roll forfurther processing, such as cutting the elongated media web 114 intoindividual printed sheets. The printer 100 depicts a simplex printerthat forms images on a single side of a print medium during a singlepass through the media path, but alternative embodiments perform duplexprinting on both sides of the media web 114.

In printer 100, each print module 102, 104, 106 and 108 is configured toeject drops of a single color of ink. For example, in a CMYKconfiguration, the print modules 102, 104, 106, and 108 eject cyan,magenta, yellow, and black (CMYK) inks, respectively. In all otherrespects, the print modules 102, 104, 106, 108 are substantiallyidentical. Print module 102 includes two print sub modules 140 and 142.Print sub module 140 includes two print units 144 and 146. Print submodule 142 includes two print units 148 and 150. In the print sub module140, the print units 144 and 146 each include an array of printheadsthat may be arranged in a staggered configuration across the width ofthe media in a simplex printer, or both the first section of web mediaand second section of web media in a duplex printer. In a duplexprinting configuration, the first section and the second section of theweb media are typically separated by a predetermined distance, and theoptical sensor 138 generates image data scanlines that include both thefirst section and second section of the media web in each scanline. In atypical embodiment, print unit 144 has four printheads and print unit146 has three printheads. The printheads in print units 144 and 146 arepositioned in a staggered arrangement to enable the printheads in bothunits to emit ink drops in a continuous line across the width of mediapath at a predetermined resolution. The print sub module 142 includesthe same arrangement of printheads in the print units 148 and 150. Thetwo print sub modules 140 and 142 provide a higher print resolution forthe print module 102, such as a 600 dots per inch (DPI) resolution forthe print module 102 when each of the print sub modules 140 and 142 areconfigured to print at a 300 DPI resolution.

In the example of FIG. 4, print sub module 140 is configured to emit inkdrops in a twenty-inch wide path that includes both the first andoptionally second sections of the media web at a resolution of 300 dotsper inch. Ink ejectors in each printhead in print units 144 and 146 areconfigured to eject ink drops onto predetermined locations of both thefirst and second sections of media web 114. A single backer roller ispositioned opposite the printheads in each of the staggered print units144 and 146, with backer roller 116 being positioned opposite theprintheads in print unit 146 by way of example. Print module 102 alsoincludes sub module 142 that has the same configuration as sub module140, but has a cross-process alignment that differs from sub module 140,so that pixels from sub module 142 are deposited mid-way between pixelsfrom sub module 141. This enables printer 100 to print with twice theresolution as provided by a single print sub module. In the example ofFIG. 4, sub modules 140 and 142 enable the printer 100 to emit ink dropswith a resolution of 600 dots per inch. As illustrated, a backer rolleris positioned opposite each set of printheads in each of the sub modulesin the printer 100.

Controller 128 is configured to control various subsystems, componentsand functions of printer 100. The controller 128 can be implemented withgeneral or specialized programmable processors that execute programmedinstructions. Controller 128 is operatively connected to memory 129 toenable the controller 128 to read instructions and read and write datarequired to perform the programmed functions in memory 129. Thesecomponents can be provided on a printed circuit card or provided as acircuit in an application specific integrated circuit (ASIC). Each ofthe circuits can be implemented with a separate processor or multiplecircuits can be implemented on the same processor. Alternatively, thecircuits can be implemented with discrete components or circuitsprovided in VLSI circuits. Also, the circuits described herein can beimplemented with a combination of processors, ASICs, discretecomponents, or VLSI circuits.

The memory 129 also stores data corresponding to a predetermineddistance between the printheads in the print modules 102-108 and theoptical sensor 138. In different embodiments, the memory 129 storesdistance data as a linear distance (e.g., a unit of microns ormillimeters), as a unit of rotational position with reference to thesignals received from the sensors 160 and 162 that indicate movement ofthe media web 114, or as units of time for different linear velocitiesof the media web 114 during operation of the printer 100. The controller128 uses the data corresponding to the predetermined distance to adjusta time at which the optical sensor 138 is activated and operated togenerate image data, and subsequently, then deactivated. The controller128 operates the optical sensor 138 to crop image data of the media web114 to close to the printed test pattern while omitting portions of themedia web that are outside of the printed test pattern from the imagedata. The controller 128 is configured to adjust the predetermineddistance data that are stored in the memory 129 to adjust the time ofoperation for the optical sensor 138 to maintain accurate generation ofthe image data including the printed test pattern. The memory 129 alsostores parameters corresponding to the range of optical detectors in theoptical sensor 138 that contain the image data of the test pattern. Thecontroller 128 is configured to crop image data generated by the opticalsensor 138 covering a portion of the image receiving surface in thecross-process direction that includes the printed test pattern.

The controller 128 monitors movement of the media web 114 with referenceto signals from the encoders 160 and 162, which generate signals inresponse to rotation of the rollers 118 and 120, respectively. Duringoperation of the printer 100, the media web 114 is propelled in theprocess direction P, and the media web 114 imparts rotation to therollers 118 and 120. The rotation of the rollers 118 and 120 producesignals in the sensors 160 and 162, respectively. In one embodiment, thesensors 160 and 162 are Hall effect sensors, and the rollers 118 and 120each include one or more permanent magnets that are located proximate tothe outer circumference of each roller. As the magnets in the rollers118 and 120 pass the sensors 160 and 162, respectively, the sensors 160and 162 generate electrical signals that the controller 128 processes toidentify the movement of the media web 114 in the process direction Pfrom the sensor signals corresponding to a rotational rate of therollers 118 and 120. In another embodiment, the sensors 160 and 162include light detectors and optical encoder discs. The optical encoderdisks are affixed to rollers 118 and 120 to rotate in conjunction withthe rotation of the rollers 118 and 120. The rotating optical encoderdisks trigger the light detectors in the sensors 160 and 162 to generatesignals corresponding to the rotation of the rollers 118 and 120, andthe controller 128 identifies the correspond movement of the media web114 in the process direction P. In addition to Hall effect and opticalsensors, alternative embodiments of the printer 100 include any sensorthat is configured to generate a signal in response to rotation ofrollers in the printer, including the rollers 118 and 120, to enable thecontroller 128 to identify movement of the media web 114.

The rollers 118 and 120 each have a predetermined diameter andcircumference. The controller 128 identifies the movement of the mediaweb with reference to the identified rotation of the rollers 118 and120. For example, in one embodiment the sensor 160 generates a signal inresponse to completion of a single rotation of the roller 118. Thecontroller 128 identifies a corresponding movement of the media web 114in the process direction P that corresponds to the predetermined outercircumference of the roller 118. The controller 128 identifies themovement of the media web 114 at the roller 120 with the signals fromthe sensor 162 in the same manner.

The printer 100 includes two rollers 118 and 120 with correspondingsensors 160 and 162 that enable the controller 128 to monitor the motionof the rollers 118 and 120, and the corresponding motion of the mediaweb 114 in the process direction. The use of two sensors at twolocations in the media path is referred to as a “double reflex” printingconfiguration. Another embodiment includes a single sensor that monitorsmovement of the media web 114 at a single location along the media path,which is referred to as a “single reflex” printing configuration. Asdescribed below, the controller 128 is configured to identify variationsin the movement and media path length for the media web 114 to enablethe optical sensor 138 to generate image data of printed test patternsusing one or more media path movement sensors, including single anddouble reflex printer configurations.

Controller 128 is operatively connected to the print modules 102-108 andcontrols the timing of ink drop ejection from the print modules 102-108onto the media web 114. Controller 128 is also operatively connected tothe optical sensor 138 to detect the process and cross-process positionsof ink drops on the media web 114 after the ink drops are ejected fromthe print modules 102-108. Controller 128 is also operatively connectedto roller velocity sensors 160 and 162 that enable the controller 128 toidentify linear speed of the media web 114 for double reflex printing(DRP). The embodiment of FIG. 4 also shows controller 128 operativelyconnected to one or more sensors, such as reflex sensor 160 and 162.

The printer 100 includes an optical sensor 138 that is configured togenerate image data corresponding to the media web 114 and a backerroller 156. The optical sensor is configured to detect, for example, thepresence, reflectance values, and/or location of ink drops jetted ontothe receiving surface by the inkjets of the printhead assembly. Theoptical sensor 138 includes an array of optical detectors mounted to abar or other longitudinal structure that extends across the width of themedia web 114 along the cross-process direction axis. In one embodimentin which the imaging area is approximately twenty inches wide in thecross-process direction and the printheads print at a resolution of 600dpi in the cross-process direction, over 12,000 optical detectors arearrayed in a single row along the bar to generate a single scanline ofimage data corresponding to a line across the image receiving surface.The optical detectors are configured in association with one or morelight sources that direct light towards the surface of the imagereceiving surface. The optical detectors receive the light generated bythe light sources after the light is reflected from the image receivingsurface. The magnitude of the electrical signal generated by an opticaldetector in response to light being reflected by the bare surface of themedia web 114, markings formed on the media web 114, and portions of abacker roller support member 156 that are exposed to the optical sensor138. The magnitudes of the electrical signals generated by the opticaldetectors are converted to digital values by an appropriateanalog/digital converter.

During operation of the printer 100, the controller 128 activates theprintheads in the print modules 102-108 to form printed test patterns onthe media web. The controller 128 monitors the motion of the media web114 and activates the optical sensor 138 to generate images for theregion of the media web 114 that includes the printed test patterns. Theoptical sensor 138 is only activated during a comparatively brief timeas the printed test pattern moves past the optical sensor so that theimage data generated by the sensor include the printed test pattern, butdo not include other printed marks on the media web. The optical sensor138 is offset from the print modules 102-108 by a predetermined distancein the process direction. Small variations in the size of the media web114 occur, however, due to media web shrinkage, media web slip and othersmall variations in the tolerances of the components in the printer 100.The variations in the printer 100 produce positional errors for thelocation of the printed test pattern in relation to the optical sensor138 when the optical sensor 138 is activated to generate the image dataof the printed test pattern. The positional errors result in the opticalsensor 138 generating image data of only a portion of the printed testpattern, or generation of images for portions of the media web 114 thatinclude printed images instead of the printed test pattern.

FIG. 3 depicts an illustrative embodiment of a test pattern 320 that isprinted on the media web 114. FIG. 3 depicts the print sub module 140,which includes print units 144 and 146, the media web 114, a printedtest pattern 320, and the optical sensor 138. The printheads in theprint units 144 and 146 form a portion of the printed dashes in the testpattern 320, while the remaining printheads in the sub module 142 andthe other print modules 104-108 form the remaining dashes in the testpattern 320. In the print sub module 140, the printhead 332 is referredto as a reference printhead.

During a registration process, the remaining printheads in the submodule 140 and optionally other printheads in the print zone are movedusing actuators to align the printheads in the cross-process directionaxis CP. The reference printhead 332, however, does not move. Instead,the other printheads in the sub module 140 and other printheads in theprint zone move, if needed, to position the printheads in cross-processregistration with the reference printhead 332 and with each other. Forexample, the printheads 334, 336, 338, 340, 342, and 344 are eachconnected to an electromechanical actuator such as the actuator 335 thatis connected to the printhead 336. The actuators adjust the printheadsin the cross-process direction to register the inkjets in the printheadsso that the inkjets can form a continuous line of ink drops extendingacross the media web 114 in the cross-process direction CP. Thereference printhead 332 does not move in the cross-process directionduring registration. In the print unit 144, an actuator 337 moves asupport member 349 that supports each of the printheads 338-344 in thecross-process direction CP.

FIG. 1 depicts a process 10 for identification of the distance betweenthe print zone, including the print modules 102-108, and the opticalsensor 138 to enable the printer 100 to operate the optical sensor 138at appropriate times for capture of printed test patterns in image data.In the discussion below, a reference to the process performing afunction or action refers to a controller executing programmedinstructions stored in a memory to operate one or more components in aprinter to perform the function or action. Process 10 is described inconjunction with the printer of FIG. 3 and FIG. 4 for illustrativepurposes.

Process 10 begins with printing of a test pattern on the image receivingsurface with a large blank region of the image receiving surfacesurrounding the printed test pattern (block 4). In the printer 100, thecontroller 128 operates the inkjets in some or all of the print modules102-108 to form a printed test pattern. In FIG. 3, the printed testpattern 320 is formed from a portion of the inkjets in the print modules102-108, including some of the inkjets from the printheads 332, 334,336, 338, 340, 342, and 344 from the print sub module 140. Duringprocess 10, the printer 100 forms the test pattern with a large blankregion surrounding the test pattern on the media web 114. In FIG. 3, themedia web 114 includes a large blank region extending at least acentimeter from the printed test pattern 320. The blank region of themedia web 114 enables the optical sensor 138 to generate image data fora large portion of the media web 114 including the test pattern 320, butnot including printed images.

Process 10 continues as the image receiving surface moves in the processdirection past the optical sensor (block 8) and the optical sensor 138is activated to generate image data for a region of the image receivingsurface with a process direction length that is longer than a length ofthe printed test pattern (block 12). In the printer 10, the controller128 operates one or more actuators to move the media web 114 in theprocess direction P at a predetermined velocity. The controller 128identifies a time at which the printed test pattern 320 approaches theoptical sensor 138 using the predetermined distance data stored in thememory 129 and the signals from the reflex sensors 160 and 162. Thecontroller 128 activates the optical sensor 138 to generate image dataof a larger region of the media web 114 that includes both the printedtest pattern 320 and blank portions of the media web 114. In FIG. 3, theoptical sensor 138 is activated and generates a series of image datascanlines covering the region 312 of the media web 114 that includesboth the printed test pattern 320 and blank margins around the printedtest pattern 320. In FIG. 3, the blank margins 356A and 356B extend fromthe region including the printed test pattern 320 in the processdirection.

During process 10, the controller 128 identifies the error in therelative process direction location of the printed test pattern in theimage data from the optical sensor 138 (block 16). When the opticalsensor 138 generates the image data without positional errors, theprinted test pattern is centered within the image data in the processdirection, with substantially equal margins 356A and 356B extending fromthe printed test pattern. During process 10, the controller 128identifies error between the predetermined location of the printed testpattern and the actual location of the printed test pattern in the imagedata that are received from the optical sensor 138. The controller 128uses one or more image processing techniques that are known to the artto identify the process direction locations of some or all of theprinted marks in the test pattern, including image processing techniquesthat are used for process direction registration of inkjets andprintheads in the printer 10.

In one embodiment, the controller 128 identifies the relative locationof one row of dashes in the image data to identify whether a processdirection offset of the scanlines that corresponds to the row of dashescorresponds to the expected scanline rows for image data in the printedrow of dashes. For example, in FIG. 3 the optical sensor 138 generatesthe image data for the downstream row of dashes 322 first during theprocess 10. The controller 128 identifies the relative process directionlocation of the first row of dashes 322 in the image data for the largerregion 312, and identifies error between the expected location of thefirst row of dashes 322 and the actual process direction location of thefirst row of dashes. In one embodiment, the controller 128 identifies anaverage process direction location for the dashes in the first row 322to reduce the effects of random noise or positional errors in individualinkjets that form the dashes in the first row 322. In anotherembodiment, the controller 128 identifies the individual processdirection location of each dash in the printed test pattern 320, andidentifies the process direction location of the printed test pattern320 as an average of the process direction locations that are identifiedfor each of the dashes. The use of an average location for the dashes inthe entire test pattern reduces the likelihood of random noise orindividual position errors in dashes from a single row of dashes, whilethe embodiment that uses a single row of dashes is faster.

During process 10, if the process direction error exceeds apredetermined error threshold (block 20), then the controller 128adjusts the predetermined process direction distance between the printzone including the print modules 102-108 and the optical sensor 138(block 24). For example, the controller 128 subtracts the identifiederror from the predetermined distance data that are stored in the memory129. The value of the identified error is positive if the actualdistance between the print zone and the optical sensor is less than thepredetermined distance value stored in the memory 129. The value of theidentified error is negative if the actual distance between the printzone and the optical sensor is greater than the predetermined distancevalue stored in the memory 129.

In the embodiment of the process 10 that is depicted in FIG. 1, theprinter 100 performs the processing described with reference to theblocks 4-24 in an iterative manner until the identified error for thelocation of the printed test patterns is less than the predeterminederror threshold. For large process direction position errors, a portionof the dashes in the test pattern may be outside the region 312 forwhich the optical sensor 138 generates image data during the process 10.A single iteration of the process 10 may correct a portion of the largererror, but when portions of the test pattern are absent from the imagedata, the process 10 may not fully correct the identified processdirection error. Additionally, the correction applied in any singleiteration may be limited in order to prevent a single erroneous readingof the test pattern location from moving the image too far in theprocess direction. Consequently, the printer 100 optionally performsprocess 10 in an iterative manner until the identified error is belowthe predetermined threshold to correct larger process direction errors.In another embodiment, the printer 100 adjusts the predetermineddistance value to correct the identified process direction locationerror once before proceeding to the processing that is described withreference to block 28.

If the controller 128 identifies that the location of the printed testpattern 320 is within the predetermined error threshold (block 20), thenthe printer 100 operates the optical sensor 138 with reference to thepredetermined distance to generate image data corresponding to a smallerregion of the image receiving surface that includes subsequent printedtest patterns (block 28). For example, in FIG. 3 the smaller region 308includes the test pattern 320 with minimal margins formed around thetest pattern 320. During a print job, the printer 100 forms testpatterns that are similar to the test pattern 320 on the media web 114in regions that are between the printed pages of the print job. Thecontroller 128 prints the test patterns and activates the sensor 138 inresponse to identifying that the media web 114 has moved thepredetermined distance from the print zone in the process direction withreference to the signals from the reflex sensors 160 and 162. Thecontroller 128 activates the optical sensor 138 to generate image dataof the printed test pattern that includes each of the marks formed inthe test pattern, while also excluding portions of the printed imagesand other marks that are formed on the media web 114. The controller 128uses the image data of the printed test patterns for registration ofprintheads and identification of inoperable inkjets in the print modules102-108 to enable high-quality image production during a print job thatincludes multiple printed pages.

FIG. 2 depicts a process 200 for identification of the individualoptical detectors in the optical sensor that should be cropped from theimage data in the cross-process direction of the image receiving surfacethat includes printed test patterns. In the discussion below, areference to the process performing a function or action refers to acontroller executing programmed instructions stored in a memory tooperate one or more components in a printer to perform the function oraction. Process 200 is described in conjunction with the printer of FIG.3 and FIG. 4 for illustrative purposes.

Process 200 begins as the printer forms a printed test pattern on theimage receiving surface (block 204), and moves the image receivingsurface in the process direction past the optical sensor (block 208). Inthe printer 10, the controller 128 operates the print modules 102-108 toform a printed test pattern on the media web 114 as the media web 114moves through the print zone and past the optical sensor 138 in theprocess direction P. The processing described with reference to theblocks 204 and 208 in FIG. 2 is substantially the same as the processingdescribed above with reference to the blocks 4 and 8, respectively, inFIG. 1. The process 10 and process 200 optionally use a single testpattern that is printed once for use in both calibration of the processdirection and cross-process direction operation of the optical sensor138.

During process 200, the optical sensor generates image data of the testpattern using a first set of optical detectors that generate image datafor a larger region of the image receiving surface than the portion ofthe image receiving surface that includes the printed test pattern(block 212). For example, in FIG. 3, the controller 128 activates theoptical detectors 352A, 354, and 352B in the optical sensor 138 togenerate image data across a full-width of the media web 114 and beyondthe edges of the media web 114 in the cross-process direction CP. Theoptical sensor 138 generates a series of image data scanlines where eachscanline includes a pixel generated by one of the activated opticaldetectors to generate image data in the region 312. In anotherconfiguration, the controller 128 activates a range of optical detectorsin the optical sensor that detect light reflected from the imagereceiving surface, but does not activate optical detectors that are pastthe cross-process direction edges of the image receiving surface. Instill another configuration, the controller 128 activates all of theoptical detectors in the optical sensor 138. In one embodiment, only theactivated optical detectors in the optical sensor 138 generate pixels inthe scanlines that form the image data including the printed testpattern and regions outside of the printed test pattern. In anotherembodiment, all of the optical detectors in the optical sensor 138generate the image data, but the controller 128 selectively ignores or“crops” the image data from the selected optical detectors.

Process 200 continues as the controller 128 identifies errors betweenthe locations of printed marks from the reference printhead in the imagedata corresponding to the test pattern and the expected locations of themarks in the test pattern (block 216). As depicted in FIG. 3, areference printhead 332 forms a portion of the printed dashes in thetest pattern 320. The reference printhead 332 occupies a predeterminedfixed location on the cross-process axis CP. The optical detectors inthe optical sensor 138 generate image data pixels corresponding to theprinted dashes from the reference printhead 332, and the controller 128identifies a cross-process direction error between the relativelocations of optical detectors that generate the image datacorresponding to the marks from the reference printhead and the boundsof the image data in the cross-process direction.

Process 200 continues as the controller 128 identifies pixels in theimage data that correspond to the identified cross-process axis errorand stores position data of the identified pixels in the memory 129(block 220). For example, in the optical sensor 138, each one of theoptical detectors generates a single pixel in each scanline of imagedata. Each image data pixel covers a predetermined length of thegenerated image data extending in the cross-process axis CP. Thecontroller 128 identifies a predetermined number of pixels thatcorrespond to the length of the identified error in the cross-processaxis CP and identifies a position of pixels extending from either end ofthe image data on the cross-process axis CP to correct the error. Forexample, the controller 128 identifies a portion of the pixels 352A thatcorrespond to the length of the error in the cross-process axis CP whenthe error for the apparent location of the printed marks of thereference printhead 332 is offset left in FIG. 3. Similarly, thecontroller 128 identifies a portion of the pixels 352B that correspondto the length of the error in the cross-process axis CP when the errorfor the apparent location of the printed marks of the referenceprinthead 332 is offset right in FIG. 3. The controller 128 storesposition information corresponding to the identified pixels in the imagedata, such as pixel column numbers or pixel range data, in the memory129. Thus, the optical sensor 138 generates image data using all of theoptical detectors in the optical sensor 138, and the controller 128identifies a region of pixels to crop from the image data to shift therelative locations of the printed marks in the test pattern along thecross-process direction axis to the expected relative location withinthe cropped image data.

Process 200 continues as the controller 128 crops the pixels in thepositions stored in the memory 129 to correct the cross-process axislocations of image data including additional test patterns that areprinted on other regions of the image receiving surface during a printjob (block 224). As depicted in FIG. 3, the controller 128 operates theprint modules 102-108 during a print job to generate additional testpatterns to maintain printhead registration and identify inoperableinkjets in the print zone. The optical sensor 138 generates the imagedata including the printed test patterns. The controller 128 cropspixels in the scanned image data to correct the cross-process directionerror that is identified above. In addition to cropping the identifiedpixels to correct the relative cross-process location of the printedmarks in the test pattern, the controller 128 optionally cropsadditional pixels from either end of the scanned image data in thecross-process axis to remove image features that are outside of theregion of the image including the printed test pattern. For example, thecontroller 128 crops image data from the optical sensors that extendpast the edges of the media web 114 on the cross-process direction axis.The edges of the media web and regions outside of the media web ofteninclude noise that increase the difficulty in reliably analyzing theimage data from the printed test patterns. Thus, the process 200 enablesthe printer 100 to generate image data that includes the printed testpattern and excludes image artifacts from other regions on and outsidethe media web 114.

As described above, the process 10 calibrates the operation of anoptical sensor to generate image data of the process direction region ofthe image receiving surface that includes a printed test pattern, andthe process 200 calibrates the operation of the optical sensor togenerate image data of the cross-process direction region of the imagereceiving surface that includes the printed test image. The processes 10and 200 can be performed in any order or concurrently to adjust theoperation of the optical sensor for detection of the printed testpatterns.

It will be appreciated that variants of the above-disclosed and otherfeatures, and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Variouspresently unforeseen or unanticipated alternatives, modifications,variations, or improvements therein may be subsequently made by thoseskilled in the art, which are also intended to be encompassed by thefollowing claims.

What is claimed is:
 1. A method for calibration of an optical sensor toscan printed marks on an image receiving surface in a printercomprising: operating inkjets in a plurality of printheads to form aplurality of marks on a first region of the image receiving surface asthe first region passes the plurality of printheads, the first regionhaving a first length in a process direction; activating an opticalsensor to generate first scanned image data corresponding to a secondregion of the image receiving surface in response to the first region ofthe image receiving surface moving by a predetermined distance in theprocess direction, the second region having a second length in theprocess direction that is longer than the first length of the firstregion and at least a portion of the first region being contained withinthe second region; identifying a relative process direction location ofthe plurality of marks in the first scanned image data with reference toa first end and a second end of the second region in the first scannedimage data; identifying an error between the relative process directionlocation of the plurality of marks in the second region and apredetermined relative process direction location within the secondregion; adjusting the predetermined distance in the process direction byan amount corresponding to the identified error; and storing a value ofthe adjusted predetermined distance in a memory to adjust a time ofoperation for the optical sensor during generation of additional scannedimage data of the image receiving surface.
 2. The method of claim 1, thegeneration of additional scanned image data further comprising:operating the inkjets in the plurality of printheads to form anotherplurality of marks on a third region of the image receiving surfacemoving past the plurality of printheads, the third region having thefirst length in the process direction; and activating the optical sensorto generate second scanned image data corresponding to a fourth regionof the image receiving surface in response to movement of the thirdregion of the image receiving surface by the adjusted predetermineddistance value stored in the memory, the fourth region having a thirdlength in the process direction that is longer than the first length andshorter than the second length.
 3. The method of claim 2 furthercomprising: identifying a registration offset between a first printheadand a second printhead in the plurality of printheads that form theprinted marks with reference to the second scanned image data.
 4. Themethod of claim 2 further comprising: identifying an inoperable inkjetin the plurality of printheads with reference to the second scannedimage data.
 5. The method of claim 1, the forming of the plurality ofmarks in the first region further comprising: operating a plurality ofinkjets in at least one printhead to form a plurality of rows of marksthat extend in a cross-process direction across the first region of theimage receiving surface.
 6. The method of claim 5, the identification ofthe error further comprising: identifying a relative location of a firstrow in the plurality of rows of printed marks in the second region inthe process direction; and identifying the error with reference to aprocess direction distance between the identified relative location ofthe first row of printed marks in the second region and thepredetermined relative location within the second region.
 7. The methodof claim 5 further comprising: identifying a relative average locationof the plurality of rows of printed marks in the second region in theprocess direction; and identifying the error with reference to a processdirection distance between the identified relative average location ofthe plurality of rows in the second region and the predeterminedrelative location within the second region.
 8. A method for calibrationof an optical sensor to scan printed marks on an image receiving surfacein a printer comprising: operating inkjets in a reference printhead toform a plurality of marks on a first region of the image receivingsurface moving past the inkjets, the first region having a first lengthin a cross-process axis; activating an optical sensor to generate firstscanned image data of a second region of the image receiving surfacewith a predetermined number of optical detectors that corresponds to thesecond region of the image receiving surface, the second region of thesurface having a second length in the cross-process axis that is longerthan the first length and the second region including the first region,the first scanned image data further comprising a plurality of scanlineswith each optical detector generating one pixel in each of the pluralityof scanlines; identifying a relative cross-process axis location of theplurality of marks in the first scanned image data with reference to afirst end and a second end of the second region in the first scannedimage data; identifying an error between the relative cross-process axislocation of the plurality of marks in the second region and apredetermined relative cross-process axis location within the secondregion; identifying a predetermined number of pixels in the firstscanned image data with a length in the cross-process axis correspondingto the identified error, the predetermined number of pixels extending inthe cross-process axis from one of the first end and the second end ofthe first scanned image data; and storing position data corresponding tothe predetermined number of pixels in a memory for use in croppingportions of additional image data generated from the optical sensorcorresponding to other regions of the image receiving surface.
 9. Themethod of claim 8 further comprising: operating the inkjets in thereference printhead to form another plurality of marks on a third regionof the image receiving surface moving past the inkjets, the third regionhaving the first length in the cross-process axis; activating theoptical sensor to generate second scanned image data of a fourth regionof the image receiving surface, the fourth region of the image receivingsurface including the third region with the other plurality of printedmarks; and cropping the second scanned image data with reference to theposition data stored in the memory to remove pixels extending in thecross-process axis from one of a first or second end of the second imagedata to adjust a relative location of the other plurality of marks incross-process axis in the second image data.
 10. The method of claim 9further comprising: identifying a registration offset between a firstprinthead and a second printhead in the plurality of printheads thatform the printed marks with reference to the second scanned image data.11. The method of claim 9 further comprising: identifying an inoperableinkjet in the plurality of printheads with reference to the secondscanned image data.
 12. The method of claim 8, wherein the predeterminedrelative location in the cross-process axis within the second region isa center of the second region in the cross-process axis corresponding toa location of the reference printhead in the cross-process axis.
 13. Aprinter comprising: a media transport configured to move a media web ina process direction through a print zone and past an optical sensor; aplurality of printheads in the print zone, each printhead including aplurality of inkjets configured to eject ink drops onto a media web; asensor operatively connected to a roller in the media transport thatengages the media web, the sensor being configured to generate a signalcorresponding to a length of movement of the media web in the processdirection; and a controller operatively connected to the mediatransport, the plurality of printheads, the optical sensor, the reflexsensor, and a memory, the controller being configured to: operate themedia transport to move a first region of the media web through theprint zone in the process direction, the first region having a firstlength in the process direction; operate the inkjets in the plurality ofprintheads to form a plurality of marks on the first region as the mediaweb moves past the plurality of printheads; activate an optical sensorto generate first scanned image data corresponding to a second region ofthe media web in response to identification that the media web has moveda predetermined distance in the process direction with reference tosignals received from the sensor, the second region having a secondlength in the process direction that is longer than the first length andat least a portion of the first region being contained within the secondregion; identify a relative process direction location of the pluralityof marks in the first scanned image data with reference to a first endand a second end of the second region in the first scanned image data;identify an error between the relative process direction location of theplurality of marks in the second region and a predetermined relativeprocess direction location within the second region; adjust thepredetermined distance of movement for the media web by an amountcorresponding to the identified error; and store a value of the adjustedpredetermined distance in the memory to adjust a time of operation forthe optical sensor during generation of additional scanned image data ofthe media web.
 14. The printer of claim 13, the controller being furtherconfigured to: continue to operate the media transport to move a thirdregion of the media web through the print zone in the process direction,the third region of the media web having the first length in the processdirection; operate the inkjets in the plurality of printheads to formanother plurality of marks on the third region of the media web as themedia web moves past the plurality of printheads; and activate theoptical sensor to generate second scanned image data corresponding to afourth region of the media web in response to identification that themedia web has moved by the adjusted predetermined distance in theprocess direction with reference to the signals received from thesensor, the fourth region having a third length in the process directionthat is longer than the first length and shorter than the second length.15. The printer of claim 13, the optical sensor further comprising: aplurality of optical detectors arranged in a cross-process axis across asurface of the media web, the optical sensor being configured togenerate scanned image data including a plurality of scanlines with eachoptical detector generating one pixel in each of the plurality ofscanlines; and the controller being further configured to: identify arelative cross-process axis location in the first scanned image data ofa portion of the plurality of marks that are formed by a referenceprinthead in the plurality of printheads with reference to a third endand a fourth end of the second region in the cross-process axis in thefirst scanned image data; identify another error between the relativecross-process axis location of the portion of the plurality of marks inthe second region and a predetermined relative cross-process axislocation within the second region; identify a predetermined number ofthe pixels in the first scanned image data with a length in thecross-process axis corresponding to the other identified error, thepredetermined number of pixels extending in the cross-process axis fromone of the third end and the fourth end of the first scanned image data;and store position data corresponding to the predetermined number ofpixels in the memory for use in cropping portions of additional imagedata generated from the optical sensor corresponding to other regions ofthe media web.
 16. The printer of claim 15, the controller being furtherconfigured to: operate the plurality of inkjets in the plurality ofprintheads to form another plurality of marks on a third region of themedia web, the third region having the first length in the cross-processaxis; activating the optical sensor to generate second scanned imagedata of a fourth region of the media web, the fourth region of the mediaweb including the third region with the other plurality of printedmarks; and crop the second scanned image data with reference to theposition data stored in the memory to remove pixels extending in thecross-process axis from one of a first end and a second end of thesecond image data to adjust a relative location of the other pluralityof marks in cross-process axis in the second image data.
 17. The printerof claim 13, the controller being further configured to: operate theplurality of inkjets to form a plurality of rows of printed marks thatextend in a cross-process axis across the first region of the media web.18. The printer of claim 17, the controller being further configured to:identify a relative location of a first row in the plurality of rows ofprinted marks in the second region in the process direction; andidentify the error with reference to a process direction distancebetween the identified relative location of the first row of printedmarks in the second region and the predetermined relative locationwithin the second region.
 19. The printer of claim 17, the controllerbeing further configured to: identify a relative average location of theplurality of rows of printed marks in the second region in the processdirection; and identify the error with reference to a process directiondistance between the identified relative average location of theplurality of rows in the second region and the predetermined relativelocation within the second region.